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With Pulsar-based navigation (XNAV) https://en.wikipedia.org/wiki/Pulsar-based_navigation , X-ray pulsar signals from a multiplicity of pulsars are used to determine the position of a spacecraft. It is somewhat analogous to GPS, where precisely timed signals from satellites in known orbits are used to determine the receiver’s position.

Since the apparent pulse rate of pulsars is practically constant and can be observed with high accuracy, measurement of the phase angle between pulses can provide accurate measures of change in radial position between the spacecraft and the pulsar.

This is analogous to the DECCA radio navigation system which used phase comparison as opposed to pulse timing, as LORAN did.

However, because the distance to a given pulsar is not known with high accuracy, only pulse arrival phase changes can be calculated, not absolute distance. In addition, there is no way to measure the absolute period of a given pulsar since the observed period will be affected by Doppler shift.

There is no privileged “correct” inertial reference system in the universe, so I assume the XNAX system initiates on whichever is the convenient inertial reference system for the spacecraft, then “counts” pulsar phase changes from the starting position to give a position with respect to the starting position.

Is this correct? Does XNAV give a position relative to the arbitrary starting inertial frame rather than in relation to the pulsars themselves?

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The frame is tied to the inertial frame of the Solar System.

The pulsars XNAV uses are detectable by ground-based radio observatories. The phase difference between x-ray and radio observations measures the difference between the distances of the x-ray and radio observatories to the pulsar.

The radio observatories are at known locations on Earth, and Earth's position and orientation in the Solar System are well determined from other observations. The reduction of the x-ray observations may bring these together in whatever solar/terrestrial frame you choose, since the relationships between the frames are known.

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  • $\begingroup$ Can you support this by linking to sources that confirm this? For example see Is NICER/SEXTANT the first civilian "spacecraft" to determine its own position in space without GPS or uplinked data? where I seem to assert (rightly or wrongly) that this demonstration was NOT tied to radio observations on Earth. "Help me Mr. Wizard!" $\endgroup$
    – uhoh
    Commented Nov 24 at 2:31
  • $\begingroup$ @uhoh sciencedirect.com/science/article/abs/pii/…, but you have to pay for it. $\endgroup$
    – John Doty
    Commented Nov 24 at 12:47
  • $\begingroup$ Is this the same document? ntrs.nasa.gov/citations/20190031975 $\endgroup$
    – uhoh
    Commented Nov 24 at 12:55
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    $\begingroup$ @uhoh It's not the same one, but it has what you want. See Figure 5. The "almanac" comes from radio observations. $\endgroup$
    – John Doty
    Commented Nov 24 at 13:09
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    $\begingroup$ @uhoh Note also that there was a dependency on GPS. NICER lacks a high stability clock oscillator. It emulates one by checking its seven low stability oscillators once per second against GPS time. In a real XNAV application, out of range of GPS, you'd need a high stability oscillator. $\endgroup$
    – John Doty
    Commented Nov 24 at 13:21

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